A molecular switch, that is. Scientists at MIT and Harvard have discovered a new class of materials that can absorb the sun’s heat and store that energy in a chemical form. These molecules, called photoswitches, can recycle that heat, and they don’t emit any greenhouse gases in the process. That’s a dramatic leap for the ever-changing solar energy industry—while these molecules wouldn’t be efficient producers of electricity, they could ignite advances in heat generation.

Molecules called photoswitches can soak in the sun's energy and make it available for later use.

The principle is simple: Some molecules, known as photoswitches, can assume either of two different shapes, as if they had a hinge in the middle. Exposing them to sunlight causes them to absorb energy and jump from one configuration to the other, which is then stable for long periods of time.

But these photoswitches can be triggered to return to the other configuration by applying a small jolt of heat, light, or electricity — and when they relax, they give off heat. In effect, they behave as rechargeable thermal batteries: taking in energy from the sun, storing it indefinitely, and then releasing it on demand.

MIT’s Jeffrey Grossman and his colleagues attached azobenzene—a type of photoswitching molecules—to carbon nanotubes in an attempt to pack the molecules as closely together as possible. Packing them closely together would give the material a high energy density, leading to a greater energy output. They didn’t expect stellar results, so what happened next came as a surprise.

Instead of hitting a projected 30 percent increase in energy density, they saw a 200 percent increase. It turned out that the key was not so much packing azobenzene molecules tightly on individual carbon nanotubes as packing the nanotubes close together. That’s because the azobenzene molecules formed “teeth” on the carbon nanotubes, which interlocked with teeth on adjacent nanotubes.

The discovery could eventually revolutionize daily life in parts of the globe where cooking fuel is wood or dung, neither of which are environmentally-friendly options. It could even prompt new methods of heating buildings—but first, the researchers will have to explore new combinations of photoswitching molecules and their substrates.

Watch this NOVA Labs video on solar power to learn more about how we can channel light and heat into energy.